Fluke Biomedical 942A-200L-M4 - Manual (Page 25)

Theory of Operation
Main Circuit Board
2
2-11
Relay Control Register (Write Only)
The Relay Control Register (U44) is an 8-bit register with clear, and responds to address 400C. Upon
initial power-up, the system reset signal sets all outputs low. Data written into U44 remains at the outputs
until are set occurs or new data is written. Table 2-6 lists relay control write functions.
Table 2-6. Relay Control Register Write Functions
Data Bit Function
D0 Fail Relay and Indicator
D1 Check Source Relay
D2 Warn Relay
D3 Alarm Relay
D4 Rate-of-rise or Aux Relay (optional)
D5 Remote Indicator
With the exception of the fail bit (D0), all outputs are applied to U48, which is an inverting open collector
driver. The outputs of U48, including fail, drive (via the J2 connector) mechanical relays located on the
relay board.
Watchdog Timer
The fail bit D0 is used as an input to U47-3 which, when set high-low-high once per second, causes
output U47-6 to remain low. Should this high-low-high sequence fail to occur (under MPU control), U47
will time out and set U47-6 high, causing the fail indicator on the panel and the fail relay to de-activate.
U48 also drives the remote indicator (when used) on the front panel.
Bargraph (Write Only)
The bargraph addresses are decoded by U5, which is a 1 of 8 decoder. Control line inputs to U5 are
/R/W, /WRITE 02, and /BARGRAPH. When the bargraph address is selected (4020-4025), U5 further
decodes the addresses listed in Table 2-7.
Table 2-7. Bargraph Write Address
Address State
A3 A2 A1
Hex
Address
Function Driver
0 0 0 4020 Green 1-10 U55, U56
0 0 1 4021 Green 11-20 U57, U58
0 1 0 4022 Green 21-24 U59
0 1 1 4023 Red 1-10 U50, U51
1 0 0 4024 Red 11-20 U52, U53
1 0 1 4025 Red 21-24 U54